Related Field
The present invention relates to direction finding and, more particularly, to correlating signals received from a signal-emitting object using antennas to determine a direction to the signal-emitting object.
Description of Related Art
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect airborne vehicles, ships, spacecraft, guided missiles, motor vehicles and terrain. The radar antenna transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to an antenna which is usually located at the same site as the transmitter.
It is also possible to use a passive technique for object detection where the antenna only receives pulses sent from other signal-emitting objects. The antenna is arranged on a platform, such as a vehicle, satellite or similar. An obvious advantage with this method is that no other signal-receiving object can track the position of the platform by means of using the passive technique for object detection. The platform can only be detected by active pulse sending radar technique.
Phase interferometry techniques in their simplest form utilize a pair of antennas disposed on a moving platform spaced apart by a known distance such that a plane wave arriving at an angle relative to the pair is received by one antenna at an earlier point in time than the other, due to the difference in path length traversed by the wave. If the signals from the two antennas are processed, their phase difference provides an indirect measurement of a direction to a signal-emitting object relative to the antenna pair.
A typical radio frequency interferometer system computes a direction to a signal-emitting object by utilizing the phase difference or phase relation of the signal-emitting object signal arriving at individual antennas of an array. The phase measurements of the interferometer can be ambiguous if the baseline, that is the separation between the antennas, is greater than half the wavelength of the received signal.
As the interferometer baseline length increases, thereby increasing the number of ambiguities, the direction measurement accuracy increases. Thus, the desire of a long interferometer baseline, conflicts with the need for robust phase or ambiguity resolution, which is easier to accomplish with a short baseline. In addition, long baselines are difficult to achieve when the receiving apparatus is on an airborne vehicle.
A typical direction finding (DF) interferometer system locates a signal-emitting object by utilizing the phase difference of the signal-emitting object signal arriving at the individual antennas. DF accuracy of such systems is directly related to DF array size which is determined by the spacing between multiple antennas of antenna array of the DF system. Simply increasing interferometric baseline without increasing the number of DF antennas leads to an increased number of ambiguities. Therefore, such prior art DF systems require many antennas and DF receivers and are very costly. The need for more antennas and more DF receivers negatively affects their use on airborne vehicle.
U.S. Pat. No. 5,835,060 entitled “Self-resolving LBI Triangulation” also teaches a long baseline interferometer (LBI) system for determining the position of a signal-emitting object. The system has two antennas and the phase differences between the signals received by the antennas at each end of the long baseline are monitored as the interferometer moves along a measurement path to obtain repetitive phase difference measurements distributed along the measurement path.
As will be appreciated, the number of antenna elements required by airborne DF interferometer systems leaves a limited amount of space for other sensors on the exterior of the airborne vehicle. Thus, it is desirable to provide a DF interferometer system for an airborne vehicle that needs only a few antennas while providing the same or greater accuracy of determining the direction to the signal-emitting object as prior art systems.